Bending of a Nitinol cantilever and its fatigue performance

The bending of a superelastic Nitinol cantilever is analyzed, where the deformation is caused by a non-parallel, rigid, friction free constraining surface that approaches the cantilever at an angle, making contact first with its tip. The analysis accounts for geometric nonlinearity and the materially nonlinear behavior of the Nitinol. The constraining surface first causes a severe bending of the cantilever, then reverses its direction of motion and allows the cantilever to partially unbend. The severe bending causes an austenite to martensite phase transformation while the unbending allows partial or full reversal of the phase change, with these transformations being associated with the materially nonlinear response of the Nitinol. In the analysis, a single-cycle of bending and unbending is considered on the one hand, and, on the other, a partial cycle of bending and unbending is followed by fatigue straining of the cantilever. To impose the fatigue straining, the constraining surface is moved up and down cyclically. The results provide the state of mean strain and strain amplitude that the cantilever will experience during fatigue straining. It is noted that recent investigations of the fatigue properties of Nitinol show that, contrary to assumptions that previously prevailed, its fatigue life is influenced by the mean strain during strain cycling.